Did they change the tooltips on the spear? I don't recall there being any mention of it using your save DC previously - especially since not all classes even have a spell save DC in the first place.
The DC usage can be tracked through the combat log, and it seems to (pretend) to use your spellcasting DC, which in BG3 is INT by default for non-spellcasting classes. I guess it was done that way temporarily, but I am not sure.
For weapon with an effect so incredibly specific, the DC should generally be a very hard one, since the item is tailor made for that single situation.
The blinding effect is separate from the extra damage effect and works on every enemy, not multi-eyed creatures only! But even with this in mind 8 is just so low... I didn't do the math, but from my testings it seems to be less than 10% on average...
